The Cardinals' win increases their lead in the second National League Wild Card spot to 2 1/2 games over the Brewers. The Dodgers are three games back after losing to the Reds.
"We've been in both games," Cubs manager Dale Sveum said. "We came back and won last night's, and had the lead with two outs in today's game. Same thing happened to them last night. We battled and had great at-bats."
Cubs reliever Jaye Chapman (0-1) issued consecutive two-out walks before Jay yanked Chapman's offering down the right-field line.
"We have to win. Everyone knows what's at stake," Jay said. "It's that time where every win is important and every game is important. It was huge."
The late-inning dramatics ensued after Marmol surrendered a solo home run to Carlos Beltran in the top of the ninth.
"I made one mistake, I paid for it," Marmol said. "That's how the game goes. He made a good swing; I take my hat off."
Marmol got Allen Craig to ground out and Tony Cruz to fly out to strand the potential go-ahead run on third. Cruz replaced catcher Yadier Molina, who left in the middle of his at-bat with back spasms after getting out of the way of a Marmol pitch.
Saturday wasn't only Marmol's first blown save since April 24, which also came against the Cardinals, but also the first home run he gave up since July 6.
"His velocity blew Jay away on three fastballs [to open the inning], and Beltran's one of the best hitters in baseball," Sveum said. "He just dropped the head on another 95 mph-plus fastball."
The Cubs had a chance in the bottom of the ninth when Josh Vitters and Luis Valbuena recorded consecutive two-out singles, but Mitchell Boggs (4-1) got Friday's hero, David DeJesus, to fly out to left.
The Cubs entered the bottom of the sixth down, 3-1, before DeJesus brought the North Siders to within one with a leadoff home run.
Darwin Barney -- who extended his errorless streak to 137 games -- was robbed of a hit on a diving play by Cardinals second baseman Daniel Descalso before Anthony Rizzo knocked a one-out single. Alfonso Soriano followed with a drive to left that took one hop before getting stuck in the ivy, forcing Rizzo to retreat to third.
After Cardinals right-hander Adam Wainwright intentionally walked Starlin Castro to load the bases, Welington Castillo hit a sacrifice fly to right field to score Rizzo and tie the game. Outfielder Dave Sappelt followed with a knock to right to plate Soriano from second to put the Cubs ahead.
"They have nothing to lose. They're just going to come here and play the game," Beltran said. "Sometimes teams just get hot or cold. They're putting together good at-bats. They're pitching well. It's not like we're facing a Triple-A team. They're in the big leagues. We just can't take them for granted."
Despite the late-inning hiccups, Cubs pitchers came through, as well. Starter Travis Wood loaded the bases in the second and allowed only one run, and St. Louis threatened in the eighth with a two-out walk and single before left-hander James Russell entered and struck out Shane Robinson.
"Woody got some big outs, and a couple other times in the game we got some big outs in crucial situations," Sveum said. "They had their opportunities, but we took advantage and came back."
A never-give-in attitude like what was on display Saturday is what the Cubs are looking for in the first year of the their rebuilding process. As shown this season and Saturday, Marmol -- one of the club's few veterans -- said the organization is heading in the right direction.
"We've got a lot of good young guys," Marmol said. "They play hard and that's what the front office is looking for."